MECH2305
Introduction to Engineering Design and Manufacturing
Casting Review
Dr Michael Bermingham, PhD
School of Mechanical & Mining Engineering, UQ, EAIT
m.bermingham@uq.edu.au
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How to we turn raw materials into final products?
Raw Material
Melting and Solidification
of primary ingots
Down stream
manufacturing
processes
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Net shape products
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(directly useful parts)
Solidification
Casting involves melting and freezing metal (solidification) but before atoms can form a solid, we need
“nucleation” (a starting ‘seed’)
In liquid, the atoms are moving
around in random patterns – no
organised structure
As the liquid cools, the atoms
come closer together and
vibrate less
Liquid – disordered structure Solid – ordered structure
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Nucleation
Casting involves melting and freezing metal (solidification) but the atoms need something to attach to
For nucleation to occur, the liquid needs
to be cooled below the freezing
temperature (this is called ‘undercooling’)
But once the first stable “seed” (nucleus
– a cluster of atoms) forms, further atoms
can continue to attach at the freezing
temperature.
Liquid – disordered structure Solid – ordered structure
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https://www.youtube.com/shorts/0kGXAkvaPnA?feature=share
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Solidification
During solidification, atoms are attaching themselves onto the existing atoms in repeating patterns (called crystals)
https://www.youtube.com/watch?v=zRUFzJrDtq0
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Nucleation
Homogenous = no ‘seed’ exists – so it takes a lot of cooling below the freezing point before solidification occurs.
Homogenous nucleation temp of water is -48°C. Practically never happens
Heterogenous = a ‘seed’ exists – some sort of platform on which atoms can attach. This may be impurities or
something deliberated added
Ultra-uniform, strong and ductile 3D printed titanium alloy through bifunctional alloy design
Jingqi Zhang, Michael J. Bermingham, Joseph Otte, Yingang Liu, Ziyong Hou, Nan Yang, Yu Yin, Mohamad Bayat, Weikang Lin, Xiaoxu Huang,
David H. StJohn, Matthew S. Dargusch, Science 383,639-645(2024).DOI:10.1126/science.adj0141
Mo particles added as a ‘seed’
during 3D printing titanium
Refer to online
lectures for more
details about how
microstructures
develop
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Solidification defects
When we pour metal into a mould, lots of things could go wrong resulting in defects
Cracking (hot tear) – caused by tensile
Porosity – trapped gas Porosity – caused by stress generated by metal shrinking when
shrinkage constrained
Inclusions (contaminations) – oxides,
slag, sand, foreign contaminates etc.
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Sand Casting
• Sand is the mould material – usually mixed with
chemicals to make it hard
• A pattern is used to create impression in the sand
• Molten metal poured into sand
• Mould is broken to extract cast part
• Can create quite complex parts by this method when
using cores
https://www.youtube.com/shorts/R6APnc0YXIo?feature=share
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Die Casting
• Steel Die is the mould material – so only lower melting
point metals that don’t react with steel can be cast
• Die needs to be manufactured, and often complex
machines used - $$$
• Molten metal can be poured in under gravity or by
pressure. In the lecture we discuss High Pressure DC
and Low Pressure DC
https://youtube.com/shorts/SDrAyixTbqs?si=xp-n71DTTA8kYVTo
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Investment Casting (lost wax casting)
• Wax pattern created → Ceramic slurry coats around the
wax to form mould → mould is sintered and wax melts
out → metal is poured in → part removed from ceramic
mould (broken out) → cleanup usually necessary (like
polishing etc.)
• Intricate parts from any metal can be cast since ceramic
has very high temperature tolerance
https://www.youtube.com/shorts/eYWK-0O-_-4
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Question & Answer
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Practice Exam Questions
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Which of the following is NOT correct?
A. Investment casting can be used to produce intricate
shapes with good surface finish from high melting point
metals.
B. It is possible for a pure metal to exist in its liquid state well
below its equilibrium freezing temperature if no
heterogeneous nucleation catalysts are present.
C. During solidification of most metals, atoms arrange
themselves into repeating patterns known as crystals
D. To make a single prototype part, High Pressure die casting
is more suitable than sand casting
E. A sand mould can only be used once
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What factor reduces the fluidity of metal
castings?
A. Increase the pouring rate
B. Decrease the viscosity
C. Reduce the superheat
D. Reduce the alloy’s freezing range
E. Reduce the liquid surface tension
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Which of the following is NOT associated
with good design for sand casting
A. Patterns should have appropriate taper to allow their
withdrawal from the sand without sand sticking to their
sides.
B. The sprue should decrease in diameter with vertical height
above the base of the sprue.
C. Pouring of molten metal into a mould should be at an
appropriate rate, sufficiently fast to ensure that complete
solidification does not occur before the mould is filled.
D. The base of the sprue should be lowest point in metal
feeding system. All subsequent metal flow should be uphill,
displacing air in a controlled progressive advance.
E. The risers/feeders must provide a sufficient pressure to be
able to feed the casting.
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Which of the following observations is NOT
a common cast defect in sand casting?
A. Hot tear
B. Shrinkage porosity
C. Foreign inclusions (for example, sand, oxides)
D. Equiaxed grains
E. Gas porosity
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Which of the following statements best describes the
functions of a riser (feeder) during sand casting?
A. It is used to supply additional molten metal to the casting to
compensate the shrinkage associated with solidification of
the casting.
B. It is used to form the shape of a casting.
C. It acts as a channel that carries the molten metal from the
sprue to the mould cavity.
D. It forms the hollow region of a finished casting.
E. It is used to carry off gases produced when the molten
metal comes into contact with the sand in the mould and
the core.
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Which of the following is NOT correct?
A. In the design of moulds for sand castings the runner is
normally included in the drag, not in the cope
B. In the design of moulds for sand castings factors including
the shape of patterns and cores, the flow of the liquid metal
and sequence of mould filling, the order of metal
solidification and shrinkage after solidification must all be
taken into account
C. Mould filling should be controlled at appropriate speeds.
Too fast metal flows lead to turbulence and entrainment of
gases and oxides, and too slow flows result in misrun and
incomplete filling
D. In order to reduce porosity in castings with non-uniform
areas or cross sections, internal and/or external chills are
normally used during solidification. The chills are placed in
regions where there is a large volume of solidifying metal
E. The design of a mould for sand casting should ensure that
final solidification occurs in the centre of the cast part to
produce an equiaxed grain structure
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Heat extraction
Chill to help
increase
solidification
rate
This large section is likely to solidify long after the thinner sections. Likely
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that the centre will contain shrinkage porosity due to lack of feeding
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Alloy Freezing range
• Different alloys have ranges that they can freeze over, the easiest way to know is to look up a “phase diagram” (we discuss
this further in later lectures)
What happens when alloy of 80% Lead and 20% tin
solidifies?
First solid to form ~93% Pb (7% Sn)
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The liquid composition is now ~66% Pb (% Sn)
Solid that is now forming is ~87% Pb (13% Sn)
First solid to form was ~93% Pb (7% Sn)
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Last pocket of liquid ~55% Pb (45% Sn)
Final solid forming at ~80% Pb (20% Sn)
Intermediate solid was forming at ~87% Pb (13% Sn)
First solid to form was ~93% Pb (7% Sn) 24
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What happens when alloy of 26.1% Lead and 73.9%
tin solidifies? (this the eutectic composition)
Liquid
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Solidification occurs at a single temperature
= Sn = Pb
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As most metals cool, they it shrinks (Al alloy)
If fresh liquid is not supplied to “fill the gaps”
then we will get a void/hole/cavity (porosity)
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Zircon (ZrO2)
Zircon flour: 45-75 µm size Zircon sand: 75-300 µm size
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Creep
Micro-voids can form at grain boundaries during creep
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shrinkage
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Squeeze casting
Combination of casting and forging
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